When you enter actual in the time sheet, it is possible to update the estimate to completion (see orange box in the image below). The estimate to completion is the amount of work remaining to complete a task.
Let's take an example : If we have a task with 20 days assigned and with no entry on the timesheet. The estimate to completion (ETC) will be 20 days.
If you enter 1 day of actual (as below in the exemple) the estimate to completion will be automatically decremented into 19 days.
As you start working on the task, you may have a better idea of the work to be done and you may estimate that the remaining work is different from the original estimate.
If for instance, you estimate that the work will need 5 more days, you can just add them to the estimate to completion column (like in the exemple below).
When the project manager reviews the project, he/she will notice that the work has been increased by 5 days. It will be up to him / her to take it into account.